Oligosaccharide is the third most abundant biopolymer in a living system, next to nucleic acid and proteins. The biological significance of oligosaccharide is undisputable, yet the rapid preparation of homogeneous oligosaccharide by automation, analogues to the synthesis of DNA/RNA oligonucleotides and peptides, remains far beyond reach.
Two of the most fundamental issues in modern chemical synthesis of oligosaccharides that requires innovation are 1) chemical glycosylation method that permits the robust construction of desired glycosidic linkage, 2) protecting groups that can be strategically applied to the blockage of designated hydroxyl, amino, carboxyl groups, yet can be readily removed to release the desired oligosaccharide. The present disclosure addresses both of these fundamental issues with respect to modern chemical synthesis of oligosaccharides.
FIG. 1 shows a comparison of known catalytic glycosylation methods with a preferred catalytic glycosylation method of the present disclosure. Currently available chemical glycosylating agents largely fall into two categories. One type is based on anomerically labile leaving groups, which can be activated by catalytic amount of a Lewis acid. The classical example is trichloroacetaimidate based glycosylating agent (Schmidt donor), but also includes glycosyl phosphite and ester-based glycosylating agent. These glycosylating agents do not tolerate acid/base treatment so that the leaving group itself has to be installed in the last step of the monosaccharide or oligosaccharide building block preparation prior to the actual glycosylation event as shown in FIG. 1. From a practical point-of-view, this is a critical drawback, as the preparation of any imidate-type glycosylating agent requires the pre-selection of a protecting group to mask the anomeric center and remove it at the penultimate step to install the leaving group.
The other type of widely used glycosylating agent is based on anomerically stable leaving group. The classical examples are thioether or n-pentenyl ether based glycosylating agents. While these types of leaving groups are anomerically stable, they have to be activated by more than stoichiometric amount of activator and require the usage of extra component, such as bulky non-nucleophilic amine base to effectively quench the in-situ generated acid.
Therefore, one would envision that an ideal type of chemical glycosylating agent should combine the catalytic activator-feature of glycosyl imidate and the anomeric stable feature of thioglycoside. Preferred glycosylation methods of the present disclosure fulfill this criterion. Moreover, the most commonly used activators in chemical glycosylation are highly oxophilic Lewis acids or thiophilic electrophiles. In both cases, the reaction will be carried out in an acidic environment, which not only calls for the extra non-nucleophilic base (not atom-economical) but also preludes the application of acid-sensitive protecting groups as permanent protecting groups in oligosaccharide assembly. The preferred glycosylation methods of the present disclosure provide a new class of thioglycoside which permits the application of cationic gold(I) complex as an activator, which is carbophilic rather than oxophilic, thus circumventing the limitation associated with the usage of oxophilic Lewis acid with conventional glycosylation agents.
Another fundamental issue in modern chemical synthesis of oligosaccharides is that too many orthogonal protecting groups for hydroxyl and amino functionalities are introduced at the early stage of the process. While the adoption of this strategy is clearly understandable, as the carbohydrate backbone contains a myriad of hydroxyls and amines which have to be “chemically protected” properly in order to achieve regioselective chain elongation, the excess orthogonalities in terms of chemical reactivity that are present in a protected oligosaccharide make the late stage chemical synthesis tedious which often results in unpredictable failure.
Benzyl ethers and ester-type of protecting groups are two most commonly used hydroxyl protecting groups in carbohydrate synthesis that requires different chemical treatment for removal. While benzyl ethers are usually sensitive to hydrogenolysis and acid, esters are sensitive to base-catalyzed hydrolysis. Within the present disclosure, it is desirable to design a series of hydroxyl protecting groups that retain the basic properties of benzyl ethers and esters, but can be deprotected by a common type of chemical reagent, acid. This aspect of the present disclosure will dramatically speed up the chemical synthesis of oligosaccharide, particularly allowing for the automation process, when coupled with a glycosylating agent that does not require strong acid for activation.